Genetic Testing for Dogs "AKC Gazette June 2001"

The Effects of Genetic Testing: Constructive or Destructive? By Jerold S. Bell, DVM, Tufts University School of Veterinary Medicine (This article originally appeared in the June, 2001 issue of the AKC Gazette; reprinted here by permission from Dr. Bell)

Every breed has genetic disorders. Finding tests that identify carriers of the genes which cause these disorders is a goal in all breeds. Once a genetic test is found, however, it is a double-edged sword: Its use can enable breeders to improve a breed or devastate it.

Without genetic tests, the number of dogs that can be identified as carriers is low, even though many dogs may be suspected of being carriers because they have relatives that are known to be affected. Without tests, though, genetic-disease control involves breeding higher-risk dogs to lower-risk dogs. Dog breeds have closed gene pools; in other words, the diversity of genes in a given breed is fixed. The number of dogs removed from consideration for breeding based on concerns regarding a specific genetic disease is usually low, and therefore does not greatly alter the breedís gene pool, or diversity.

However, once a genetic test is developed that allows breeders to positively determine if a dog is a carrier of a defective gene, many owners are likely to remove carrier dogs from their breeding stock. Although doing so is human nature, this temptation must be overcome. Any quality dog that you would have bred if it had tested normal should still be bred if it tests as a carrier.

Okay, so I did not post this information to say that you shouldn't test your animals. I have just recently been doing a lot of research into this because I knew so little about it. The reason for this post was to give others like me an example of what to do with the test results. i.e. breeding non carriers with carriers vs. spay/neuter the carriers and limiting the gene pool. I know there are several of you out there with more knowledge than I about these matters, is this how you run your breeding program, or is this the opposite of what you have been taught/believe?

Traits that are largely under the control of genes (little environmental influence) are said to be highly heritable. This is the situation where puppies tend to be more similiar to their parents with respect to a certain trait. Selection of dogs (making planned matings to change the average value of a trait) works best when traits being selected are highly heritable. Improvements in highly heritable traits can be accomplished more quickly than in traits that are lowly heritable. Generally, traits such as mature size and conformation are highly heritable. Traits that are not highly heritable are heavily influenced by non-genetic factors, or environmental influences. Selection for these traits is usually not very successful, or is very slow. Traits with low heritability are also traits that tend to show high levels of heterosis, or hybrid vigor. Examples of lowly heritable traits would be those associated with reproduction and survivability. Hybrid vigor has occurred when the performance of the offspring is different from that of the average of their parents. The actual mechanism of hybrid vigor is not well understood, however, matings can be made to increase the beneficial effects of hybrid vigor. Mating of animals which are not alike in their pedigree or that are not related are those which will generally exhibit higher levels of hybrid vigor. Closely related animals do not exhibit high levels of heterosis. Crossbreeding in plants and animals is done to take advantage of both hybrid vigor and complimentarity. Complimentarity is the term used to describe the "combining" of desirable traits from two breeds or lines that are not related. Hybrid vigor is also maximized when animals are mated that have little or no genetic history in common. When animals that are related are mated together, the offspring are termed "inbred". Inbreeding in animals will eventually lead to the reduction in vigor of offspring, lowered productivity and fertility, and will increase the incidence of genetic defects. Common inbreed matings include brother-sister matings and father-daughter or mother-son matings. Inbreeding, like crossbreeding, is a form of mating, but inbreeding and crossbreeding are at opposite ends of the mating system spectrum in terms of genetic variability. Crossbreeding increases variability and close inbreeding, generally, reduces variability in the progeny.

It seems to me the terms "genetic testing" and "health testing" get used interchangably on this board, which is making for a lot of confusion. Both on this thread and on the "breed/breeder-bashing" one. The articles here are talking about the tests that analyze genetic material for some extremely specific traits, and the articles discuss whether and how you might decide to breed an animal that's found to be carrying a bad trait. Carrying, not afflicted with. This is very different from the OFA and other tests that are about whether dogs actually have a condition, and if so, how bad they have it. That's the kind of testing we normally talk about here, and it's pretty clear why it's a responsible thing to do. It doesn't have much to do with the actual genetic tests these articles talk about.

The first article is definitely about taking a dog out of a breeding program because of carrying the genes for these diseases. I am just curious. I know very little about the testing everyone keeps talking about. I asked questions and I started doing the research so I would know more about it. I am trying to educate myself on these issues. Everywhere I looked, I kept coming up with these articles and I am asking about them.

I totally agree that screenings for eyes, hips, thyroid, heart, and other breed appropriate screenings are helpful. I have a few practical questions that I would like to have answered. If you have a stud who has really good confirmation, is in following with all the breed standards, does not HAVE a genetic disease, but he is a carrier, would you have him neutered, or just show him without breeding him, or breed him anyway because his good traits outnumber the bad, or breed him, but only with females that are not carriers?

The next question, if you have a pair of breeders, and they have been screened and are negative for everything, not even carriers, and a puppy buyer calls you and says their puppy has a genetic disease, lets just say going blind. Would one genetic mutation be enough to stop using those breeders? Would you tell new puppy buyers about the one puppy?

Ok so each point has more than one question. I am just trying to learn.

1. From the sounds of this it seems as if you are asking about PRA? If that's the case, can you tell me, and I will give you specific answers on that. Or, are you referring to something else. Because there are so many problems we need to narrow down, if possible, which problems you are referring to. Heart/PRA/etc..

2. EXCELLENT question. But, again, it depends. Are we speaking about a hip problem, elbow problem, heart problem? The reason I ask is because one may think it's genetic, but it's not. Example, joint problems,, did you know that the majority of problems in puppies with joint problems (pending they come from good lines of course) is NOT a genetic problem but ownership, over exercising!! So again I ask just a little more information and can give you specific answers.

I answer to a couple of your questions, Janders,I can speak for my breed, because that's the one I know best. The first two articles were good, but I disagree totally with the Continental Kennel Clubs statement that common inbreeding is done between brother- sister, father-daughter,mother-son. I have never, nor would I consider such a breeding, and I don't know any breeders who would. Maybe it's done, but no reputable breeder would consider it. As for your question about the stud being a carrier, this has happened to me. At the age of 1 one of my boys eyes were pronounced clear for cataracts. At the age of 2, he was tested again, after gaining his championship, and being finished in show.The opthamologist tested him and noticed a pin sized mark, and my heart sank. I had him D.N.A tested, and he is in fact a carrier for cataracts.if I wanted to I could breed him, but only to a bitch that has been tested negative as a carrier. I still have not made up my mind about this, as even though the science says it can be done, I don't altogether trust the exceptions to the rules that can crop up, especially with my dogs. In answer to your question about having parents who produced a blind puppy, I can't really comment, because I don't know the breed. I guarantee my puppies to be free of juvenile cataracts, and seizures, and that's all. But health guarantees are breed specific; so I can't comment on other breeds. I will not breed any of my studs to a bitch who has not been OFA'd for patellas, or DNA tested for cataracts.As far as honesty goes, if you have a known defect,you don't breed those dogs, thats the way I breed. That is why I'm having a tough time with my champion male.Many of my breeder friends say I'm silly not to breed him, but I'm still not sure.

Pen, pm me, interesting story for you on cataracs. I've experienced that one too. I think as long as we have been doing this, we see it all.. although just when you think you've had it all or seen/heard it all BAM BAM there it goes and something new UGG

I think the reason I want more information is because the questions are good and so much is subjective.. example, 2 OFA Excellent dogs (hips) elbows clear and you still have a puppy that has a problem... Then, PRA.... that's absolute as it's a DNA test and even if you have a carrier, you can still properly breed and NEVER ever produce an affected dog. Janders will let us know. :-)

Well they were just hypothetical questions. There is no dog to get specifics from as I am not breeding any dogs. The only specifics I know is that with mini Schnauzers there are 2 tests that I would definitely want to run before I ever made a decision to breed them VWD and liver shunts. Every good breeder I have talked to has specifically listed those. Savannah, you actually hit the nail on the head in one of your replies. Some of these tests are not genetic. And if the dog has them, it may not pass it along. So the big question then would be if it would cause undo stress if it was a female to have a litter. Pen, since that dog has done so well in the ring, I think I might find a clear female. It is a hard call, but the breed definitely does not need to lose good genes. And it sounds like he has them. All great answers. Thanks guys! This has been a great help to me!

The stress I was referring to would be (and again I have no experience with this because I have never owned a dog that had this problem) like a hip disorder or something that would put more than average stress on her. OFA? And again, I am asking because everyone talks about these tests and what to do with the results and I don't know. I am trying to learn. If I ask something that seems silly, please just chalk it up to innocent ignorance.